1. Field Of The Invention
The present invention relates to vacuum pumping systems and, more particularly, to a vacuum pumping system powered by a single drive motor and having two pump stages in series.
2. Summary Of The Invention
During the evacuation of a container with a two-stage vacuum pumping system powered by a single drive motor, the load on the motor is at a maximum during the initial pull-down of pressure from the ambient and the power requirements lessen substantially during the subsequent pump down of the container. Because of these differences in power requirements, the selection of an efficient drive motor to meet the needs of a conventional vacuum pumping system poses many difficulties. If a motor adequate to meet the requirements of pull-down is installed, there is a danger that the system will be inefficiently overpowered during pump down. However, to avoid overloading the motor destructively, it is conventional practice to install a motor that is larger than is required for final pump down. This consequently results in a higher first cost of the system and also an overall drop in efficiency of the installation during operation. In the subject invention, a pressure controlled bypass is provided in the interstage line between the two series-connected stages of the vacuum pumping system; excess discharge pressure from the first stage pump is relieved back to the container being evacuated to thereby reduce the load on the motor during the pull-down phase. Thus, a smaller motor that is energy-efficient during the final pump down phase can be used to drive the vacuum pumping system.